316 ALIMENT consumes rather more than a quarter of a pound of albuminoid matters (calculated in the dry state) during 24 hours. No one of the groups of alimentary materials enumerated above, taken singly, is sufficient for the contin- ued nourishment of the body. This is sufficiently evident for the inorganic substances, such as water and the mineral salts. Vegetables have the power of assimilating these matters, and converting them into the ingredients of the vegetable fabric ; but animals require for their nourishment materials which are already ani- mal or vegetable in their nature. But even these substances, combined with starch, sugar, or oil, are also insufficient. Dumas and Milne- Edwards found that bees fed upon pure sugar and water soon ceased to work, and afterward perished. They thrive only when supplied with waxy and other vegetable substances in addition. Magendie found that dogs fed upon starch and sugar, or upon an exclusive diet of fat, became after a tune debilitated, and died with symptoms of great disturbance of the nutritive functions. Boussingault fed a duck upon butter alone ; but, although the quantity of this alimentary substance was abundant, namely, from 1,350 to 1,560 grains per day, the animal at the end of three weeks died of inanition. All the tissues of the body were infiltrated with oleaginous material, but this substance had proved incapable of supporting life. Lehmann put himself upon a regimen consisting solely of non-nitrogenous substances, such as starch, sugar, gum, and oil, but was only able to continue this course for two or at most for three days at a time, owing to the dis- turbance of the general health produced by it. The unfavorable symptoms, however, rapidly disappeared on his resuming an ordinary mixed diet. The substances just mentioned being de- ficient in so important an element as nitrogen, this was at one time regarded as a sufficient explanation of their inability to sustain life ; and the albuminoid or nitrogenous materials were therefore supposed to be the only ab- solute and completely nutritious ingredients of the food. Direct experiment, however, showed that these substances themselves, when taken alone, were also insufficient. Magendie fed dogs upon pure gelatine and pure fibrine, and found at the end of some days that the animals lost their relish for the food, became emaciated, and died with symptoms of inanition. To be completely nutritious, therefore, the food must contain not one but all of the groups of ali- mentary substances, and these substances must be present in their true proportion. This shows the futility of the attempts which have some- times been made to fix the nutritive value of different kinds of food by ascertaining their ultimate chemical composition, and particularly by the amount of nitrogen which they contain. The nutritious qualities of an article of food depend upon the proportion of its different in- gredients, not only as taken alone, but also as used in combination with other substances. Its digestibility and the extent to which it con- forms to the appetite and natural taste are also important elements in the question. The nu- tritive value of an article can therefore only be determined by direct experiment and observa- tion ; that is, by employing it as food, alone and in combination. Thus all those sub- stances which are found by universal expe- rience to be the most useful are distinguished, by a variety of composition. Milk, which for the young infant is during a certain period the only food employed, contains water, mineral salts, caseine or an albuminoid ingredient, but- ter or fat, and a peculiar variety of sugar. Eggs contain albumen, water, fat, and salts. "Wheat flour, as well as the bread which is made of it, contains gluten, water, salts, starch, and a small quantity of sugar. In practice, at least for adults, a judicious variety in the diet is found to be indispensable for the maintenance of health. Of all articles of food, bread is per- haps the most important. The best and most nutritious bread is that made from wheat flour. The flour contains, in 100 parts, on the average, 72 parts of starch, 7 T parts of gluten, 5 T % parts of sugar, and 12 parts of water, together with gum, phosphates of lime and magnesia, alkaline sulphates, and a little chloride of sodium. It is first kneaded. into a paste with about one half its weight of water, a little yeast added and thoroughly mingled with the mass by con- tinued kneading, and the dough then allowed to remain for some hours at a moderately warm temperature. During this time the yeast ex- cites in the sugar of the flour a fermentation, by which it is converted, as in ordinary fer- mentation, into alcohol and carbonic acid. The alcohol penetrates the dough and escapes by evaporation. The carbonic acid, however, is developed throughout the dough in the form of minute gaseous bubbles, which are confined and entangled by the tenacious gluten of the flour ; and the whole mass thus becomes in- flated or puffed up by the gaseous expansion. This is the rising or fermentation of the dough. It is then transferred to an oven and kept there at a temperature of about 380 F. until the baking is complete. The effect of baking at this high temperature is as follows : First, the starch upon the outside of the loaf is converted into dextrine and hardened -into a brownish, brittle layer, which is the crust ; secondly, the gluten throughout is also solidified and at the same time acquires an agreeable and whole- some flavor ; thirdly, the starch grains become swollen, fused, and hydrated, fixing perma- nently in their substance a certain proportion of the water with which the flour was mingled. Thus, after baking, the bread always weighs more than the flour of which it was made, owing to the necessary combination of water with the starch in the baking process. Usually one pound of flour is found to produce in this way one pound and a quarter of bread. "When removed from the oven and cut open, the in- terior of the loaf is seen to present a spongy
